1. Field of the Invention
The present invention relates to a method for processing half tone image data.
2. Related Art
As well known to those skilled in the art, in a data transmitting machine such as a facsimile machine, data to be transmitted are compressed at the transmission side to reduce or shorten a communication time and the compressed data are expanded at a receiving side.
The data compression and expansion is performed under a predetermined rule such as MH method, MR method or the like which is designed so as to be able to transmit usual documents containing characters and figures effectively. Namely, in such an ordinal document, such a probability that white or black dots occur in succession is very high and, therefore, a lump of signals representing successive dots of the same kind is transformed to a signal having a number of bits smaller than the sum of bits of individual original signals. Although data having a low probability of occurrence which is comprised of a relatively small number of dots is transformed to a signal having a larger number of bits inconveniently, the efficiency of data compression is much improved as a whole.
Recently, it is highly demanded to transmit documents including half tone images such as pictures and there have been proposed facsimile machines having a picture mode for transmitting half tone images faithfully.
Upon processing half tone images, according to a conventional method, for representing half tone images each picture element is represented by plural dots having a two-dimensional spread, for example in a matrix form and the density thereof is represented by a ratio of black and white dots contained therein.
In the known data compression method, there is a possibility such that the efficiency of data contraction may be decreased upon transmitting half tone images since they have a low probability of occurrence of successive dots of the same kind when compared with a usual document including characters mainly.